Circuit and method for terminating a write to a memory cell

ABSTRACT

A start write sensing circuit for sensing a start of a write is coupled to a write simulation circuit. The write simulation circuit preferably includes a memory cell replicate to mimic the amount of time required for writing data to the memory cell. The state of the data stored in the memory cell replicate is changed upon the write sensing circuit sensing the start of a write. The memory cell replicate is preferably constructed using the same structure, design, and process as the memory cells of the array so as to accurately simulate the time required for writing data to a memory cell in the array. Upon the write to the memory cell replicate being completed, a write termination signal is generated for terminating the write signal. The write termination signal also is a reset signal for resetting circuits of the array to prepare for the next cycle, whether it be a read or a write.

This application is a continuation of U.S. patent application Ser. No.08/588,737, filed on Jan. 19, 1996, now abandoned.

The following pending U.S. patent applications Ser. No. 08/582,424, byDavid C. McClure entitled: "Architecture Redundancy," and "RedundancyControl," Ser. No. 08/580,827, which were filed on Dec. 29, 1995, andhave the same ownership as the present application, and to that extentare related to the present application, which are incorporated herein byreference; and entitled: "Test Mode Activation And Data Override," Ser.No. 08/587,709, "Pipelined Chip Enable Control Circuitry AndMethodology," Ser. No. 08/588,730, "Output Driver Circuitry Having ASingle Slew Rate Resistor," Ser. No. 08/588,988, "Synchronous StressTest Control," Ser. No. 08/589,015, "Write Pass Through Circuit," Ser.No. 08/588,662, "Data-Input Device For Generating Test Signals On BitAnd Bit-Complement Lines," Ser. No. 08/588,762, "Synchronous OutputCircuit," Ser. No. 08/588,901, "Write Driver Having A Test Function,"Ser. No. 08/589,141, "Circuit And Method For Tracking The Start Of AWrite To A Memory Cell," Ser. No. 08/589,139, "Circuit And Method ForTerminating A Write To A Memory Cell," Ser. No. 08/588,737, "ClockedSense Amplifier With Word Line Tracking," Ser. No. 08/587,728,"Memory-Row Selector Having A Test Function," Ser. No. 08/589,140,"Synchronous Test Mode Initialization," Ser. No. 08/588,729, "Device AndMethod For Isolating Bit Lines From A Data Line," Ser. No. 08/588,740,"Low-Power Read Circuit And Method For Controlling A Sense Amplifier,"Ser. No. 08/589,024, and "Device And Method For Driving A ConductivePath With A Signal," Ser. No. 08/587,708, and by Mark Lysinger entitled:"Burst Counter Circuit And Method Of Operation Thereof," Ser. No.08/589,023, all of which have the same effective filing date andownership as the present application, and to that extent are related tothe present application, which are incorporated herein by reference.

FIELD OF INVENTION

This invention relates to a method and apparatus for controlling a writeto a memory cell, and in particular, for terminating a write at aselected time after the write has started.

BACKGROUND OF THE INVENTION

Computer processors are increasingly becoming more complex and capableof performing many tasks simultaneously. At the same time, computerprocessors are now operating at higher speeds and able to input andoutput greater amounts of data. At the same time, the voltage level ofwhich computer processors and other devices has decreased. For example,many computer processors today use a full 32 bit bus for data input andoutput while operating at very high speeds. The voltage requirement isalso reduced from 5 volts to lower voltages, such as 3.3 volts, arelower. There is therefore a need for memory devices which are capable oftransferring 32 bits of data simultaneously while at the same timeoperating at very high speeds and at lower voltage levels. The highspeed of operation is required not only for reading data from the memorydevices but also for writing data to the memory devices under control ofthe microprocessor.

At low supply voltages, for example 3.3 V and lower, correctly writingdata to a memory cell is subject to more sensitivity than at highersupply voltages, such as 5 V. In addition, writing to memory cells at alower voltage may require more time than writing at a higher voltage andmay also require more time than reading data from the same memory cell.

At lower voltages, the body effect of a transistor has a greater affecton the operation of the transistor itself. In addition, the thresholdvoltage becomes a higher percentage of the total supply voltage whenoperating on a lower supply voltage. Because of these, and many otherfactors, the voltage available for storing data using a lower supplyvoltage is much less than is available when operating at the highersupply voltages.

A further problem with operating at lower voltages is that the speed atwhich data can pass through a transistor while the transistor is beingturned ON is reduced because both the gate voltage and the data voltageare reduced from that which would be available at a higher voltage.

Even minor variations from one memory device to another may affect thetime required to store data in seemingly identical memory cells. Forexample, even using the same manufacturing process, the bit lineresistance may vary from one lot to another. Further, the contactresistance, whether buried contact or via, may vary slightly from onelot to another. Variations in the properties of the active areas, suchas the bird's beak, the threshold voltage and the body effect may alsocause variations in the time required to write data to a memory cell. Atlower voltages, even very slight changes in any one of these parameterswill affect the speed at which data can be correctly written to thememory cell.

According to the prior art, the time permitted to perform a write to amemory cell was established according to the product specifications fora given product design. Enough time was provided to ensure that data waswritten to the properly manufactured memory cell. If data cannot becorrectly written to the memory cell in the time established by thespecifications, the entire memory chip is deemed defective and cannot besold. By establishing the product specifications at a long write time,the number of defective chips is reduced. However, this results in aslower speed of the memory devices. As faster and faster computerprocessors become available, the speed at which memory can be accessedis an important parameter for the commercial success of the memory onthe market. It is therefore desirable to provide a memory in which datacan be written to as quickly as possible while at the same timeproviding a memory which meets product specifications so that the chipsdo not need to be discarded.

SUMMARY OF THE INVENTION

According to principles of the present invention, a write sensingcircuit is provided is provided for sensing the start of a write to amemory cell. A write time duration circuit is coupled to the writesensing circuit and receives a signal at the start of a write. At aselected time after the write start signal is received, the write timeduration circuit outputs a signal indicating that sufficient time ispast for a write to have been completed to memory cells in the array.

A write termination signal generation circuit is connected to the writetime duration circuit and receives the signal indicating that sufficienttime has past for a write to be completed. Upon receiving the writecompletion signal, a write termination signal is generated. The writetermination signal also acts as a reset signal. The write terminationand reset signal is received by a number of circuits associated withaddressing and accessing the memory cell so that write is terminatedimmediately upon the generation of the signal. In addition, the circuitsare reset so that they are in a state for receiving of subsequent writeor read command immediately after the reset signal is received.

According to one embodiment of the present invention, the write timeduration circuit is a write simulation circuit which simulates theamount of time required for a memory cell to change data state. Thesimulation is obtained by having a memory cell replicate within thewrite simulation circuit. The memory cell replicate has similarconstruction and sizing of transistors as in the memory cells within thearray. The same amount of time for writing data to the memory cell ofthe array is also required for writing data to the memory cellreplicate.

Upon the write sensing circuit indicating that a write is beginning, thewrite simulation circuit causes the data stored in the memory cellreplicate to change state. Immediately upon the state of data in thememory cell replicate switching state, the write simulation circuitoutputs a write time completed signal to switching logic. The switchinglogic inputs a signal to the reset logic which generates the writetermination and reset signal.

According to one embodiment of the present invention, the write sensingcircuit senses a change in state of the data being written to the memorycell. It is the changing state of the data itself which is sensed toenable the write simulation circuit to start the simulation of the timerequired to write to the memory cell. It is the changing of the databeing written to the memory cell which also causes the memory cellreplicate to change the state of the data stored therein. The memorycell replicate completes its change of state at approximately the sametime that the write to the memory cell is completed because thestructure of the two memory cells are identical and they are formedusing the same process on the same very same die. The completion of thechange of the state of the memory cell replicate can therefore bereliably used to force a termination of the write and therefore disablethe write related circuits and enable other circuits to perform asubsequent read or write, depending upon the next instruction from themicroprocessor. In an alternative embodiment, the write sensing circuitsenses other features, such as a transition in the least significant biton an address bus or other circuit transition which is accurately timedagainst when the write begins to occur to the memory cell. The memorycell replicate receives such a signal and, because its design structureis so similar to that of a memory cell in the array, it accuratelyreflects when the write may be terminated.

The invention advantageously permits accurate and exact timing of when adata write can be terminated so that completion of the write to memorycells occurs much more quickly than was previously possible in the priorart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a memory device having a memory array thereon.

FIG. 2 is a block diagram of various circuits on the memory device ofFIG. 1.

FIG. 3 is a block diagram of additional circuits on the device of FIG.1.

FIG. 4 is a circuit schematic of a memory cell.

FIG. 5 is a block diagram of the reset control circuit of FIG. 2.

FIG. 6A is a detailed schematic of the data input buffer as shown inFIG. 2.

FIG. 6B is a detailed schematic of the write driver circuit as shown inFIG. 2.

FIG. 6C is a detailed schematic of the column select circuit of FIG. 2.

FIG. 6D is a detailed schematic of the sense amp and global bus driverof FIG. 2.

FIG. 7 is a detailed schematic of the block read/write control of FIG.2.

FIG. 8A is a detailed schematic of one embodiment of the block resetcontrol circuit of FIG. 5.

FIG. 8B is a detailed schematic of a second embodiment of a blockread/write control circuit with write timing control circuitry.

FIG. 9 is a detailed schematic of one embodiment of a fast write circuitas shown in FIG. 8B.

FIG. 10 is a detailed schematic of one embodiment of a slow writecircuit of FIG. 8B.

FIG. 11 is a detailed schematic of a write timer test mode and speedselection circuit.

FIG. 12 is a detailed schematic of one alternative embodiment of a writesensing circuit.

FIG. 13A shows the voltage versus time at various nodes in FIG. 8A.

FIG. 13B shows the write timing control circuitry at various nodes ofFIG. 8B.

FIG. 14 is a block diagram of a system according to the presentinvention.

FIGS. 15A and 15B are block diagrams of alternative embodiments of asystem using the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a memory device 50 having a memory array 52 thereon. Thememory array 52 is subdivided into a plurality of memory array blocks54. The memory array is subdivided into as many memory array blocks asdesired, according to the design. For example, eight blocks, nineblocks, or 16 blocks are rather common numbers of array blocks. In oneembodiment, 32 memory array blocks 54 are formed, as is shown in FIG. 1.The 32 blocks are grouped into four quadrants; each quadrant havingeight blocks. There are four quadrants on the memory device 50.

Associated with each memory array block 54 is the respective blockinput/output circuitry 56 and word line drive circuitry 58. In oneembodiment, the word line drive circuitry 58 for two blocks of memory ispositioned in a single region between the two adjacent memory blocks 54.Alternatively, it can be located in the central or peripheral regions ofthe device 50. Other circuitry for accessing the cells in the array,such as the row and address decoders, input/output buffers and senseamplifiers are located in the block I/O circuitry 56, the centralregions 60 and 62 and other positions on the device 50 as needed. Aplurality of bonding pads 64 are provided in the peripheral region ofthe chip for connecting to data input/output pins, voltage supply lines,address lines and other electrical connections as needed for the memorydevice 50.

FIGS. 2 and 3 illustrate block diagrams for various circuitry as foundon the memory device 50. Each block of memory array 54 is provided withcircuitry for providing data to and from for that individual block. Inone embodiment, the circuitry of FIGS. 2 and 3 will be provided for eachblock 54 so that there are 32 such circuits on a single device 50.Alternatively, for that circuitry which can be shared between two blocks50, only 16 such circuits will be needed, as will be apparent to thoseof skill in the art. In one embodiment, the memory device 50 is capableof receiving 32 bits of data simultaneously and outputting 32 bits ofdata simultaneously. Therefore, all circuitry required to input andoutput 32 bits of data simultaneously is provided, such as 32input/output buffers, and the like. The 32 bits can be provided bysimultaneously accessing one memory cell in each of the 32 blocks or,alternatively, by accessing 8 memory cells in one block within onequadrant and accessing four blocks one within each quadrantsimultaneously. The circuits shown in FIG. 2 and FIG. 3 are thusprovided for each individual block of memory array 54 and can have a 1bit bus, an 8 bit bus, a 4 bit bus or the like.

As shown in FIG. 2, a data signal line 27 receives data and provides thedata to input buffer 68. The data input buffer 68 outputs the datacomplement DC, on a signal line 70 and the data true DT, on a signalline 72. The input buffer is shown and described in more detail withrespect to FIG. 6A.

A write driver 75 receives the data and outputs the data on signal lineswrite bit complement, WBC 74 and write bit true, WBT 76. The data inputbuffer also outputs the written data to an output buffer 98 on line 97.

The signal lines WBC 74 and WBT 76 are input to a column selectcircuitry 78. The column select circuitry 78 outputs the data on bitline complement BLC 80 and bit line true BLT 82 for writing to thememory array. The BLC line 80 and BLT line 82 are connected to thememory array block 54 as shown in FIG. 3. The WBC and WBT signal lines74 and 76 are also connected to a reset control circuit 84 which outputssignal lines reset 86 and reset bar 88. Reset control circuit 84receives additional input signals as explained in more detail withrespect to FIGS. 5, 8A and 8B.

The write driver circuit 75 also receives additional input signals asexplained in more detail with respect to FIG. 6B.

The column select circuit 78 also receives additional input signals tocontrol reading and writing data to and from the memory array block 54as explained in more detail with respect to FIG. 7.

Read bit complement RBC 90 and read bit true RBT 92 signal lines areoutput by the column select circuitry 78 and carry the read bit datawhen the circuitry is in the read mode. RBC 90 and RBT 92 are input tothe sense amp circuitry 94 which also receives additional signals asexplained in more detail with respect to FIG. 6C. Sense amp circuit 94outputs signal lines RBC 90 and RBC 92 to a global bus driver circuitry95. A global drive bus signal line, GDB 96 is connected to the globalbus driver 94 and also connected to an output buffer 98 to provideoutput of the data on data line 27. A read/write control circuitry 125receives a number of input signals and outputs additional signals tocontrol the reading and writing of data to and from the memory arrayblock 54 as explained in more detail with respect to FIG. 6C.

FIG. 3 illustrates block diagrams of additional circuitry associatedwith the memory array blocks 54. In particular, an address decoder 102receives address information and outputs address information to the wordline and block select latch circuit 104. Additional address decodecircuitry including an input buffer 106, an even/odd row address driver108 and a word line select circuit 110 are part of the address decodecircuitry. The word line select circuitry provides signals to the localword line driver circuitry 112 which outputs signals to drive theindividual word lines of the memory array block 54 as shown in FIG. 3.As will be appreciated, the appropriate address decoder circuitry forthe column address is also provided so that individual memory cells areaccessible. Test mode logic 114 is also provided to permit testing ofthe memory device 50. Any acceptable circuits for the blocks of FIG. 3may be used as is known in the art, the structure and operation of thesecircuits is not part of the present invention.

FIG. 4 is a circuit diagram of a memory cell within the memory arrayblocks 54 for storing data according to one embodiment of the invention.In this embodiment, the memory cell is a four-transistor static memorycell and the device 50 is an SRAM. The SRAM memory cell includes twocross-coupled transistors 202 and 204 and two access transistors 206 and208. A bit line BL 210 is connected to one access transistor and a bitline complement BLC 212 is connected to the other access transistor. Therespective drains of the cross-coupled transistors 202 and 204 areconnected to voltage supply through poly resistors 214 and 216. Memorycells of this type are well-known in the art and any current techniqueand process for forming such memory cells is acceptable. In analternative embodiment, the memory cell according to the invention is aDRAM memory cell having a single access transistor and a capacitor forthe data storage node. In further alternative embodiments, the memorystorage device is a non-volatile memory, such as an EPEPROM, a flashEEPROM or the like.

Data is stored at nodes 218 and 220, node 218 being true data and node220 being complement data. During a write, bit lines 210 and 212 mustalways write opposite data states to the memory cell, one bit linewriting true data and the other bit line writing complement data to therespective true and complement storage nodes.

FIG. 5 is a more detailed block diagram of the reset control circuit 84as shown in FIG. 2. The reset control circuit 84 outputs reset signal 86and reset bar signal 88 to signal termination of a write to a memorycell as explained in more detail herein.

The reset control circuit 84 receives signals from other circuits in thearray in order to generate reset signal RESET 86 and reset bar signalRESET B 88. In one embodiment, as shown in FIGS. 8A and 8B, the resetcontrol circuitry 84 contains circuitry for two memory array blocks 54and thus can control the resetting of two adjacent memory blocks 54. Inan alternative embodiment, one reset control circuit 84 is provided foreach block of memory as shown in FIG. 5. In a further alternativeembodiment, one reset control circuit is provided for each 8 blocks ofmemory so that one quadrant of the memory device 50 is controlled by oneblock. Further, the entire chip can be controlled by one reset block sothat the termination of the write cycle for an entire chip is controlledfrom a single circuit according to the invention. In preferredembodiments, the reset control circuitry is provided for each memoryarray block 54, or alternatively for each pair of memory array blocks,so that each block of memory is individually controlled for terminationof a write to a memory cell within that particular block. The resetcircuitry 84 includes forced reset logic 116, write sensing circuit 118and write simulation logic 120. The write simulation circuit 120 outputis connected to the switching logic 124 which is connected to the timecontrol circuit 126. The time control circuit 126 is connected to thereset logic 128 which ultimately outputs reset signal 86 and reset barsignal 88.

FIG. 6A shows a detailed schematic of the data input buffer 68 from FIG2. As shown, the data input buffer 68 receives data on line 27 andincludes input circuitry 234, delay circuitry 236, a latch circuit 238,a test logic circuit 240, a first driver 242, and a second driver 244.As shown, the input circuitry 234 receives the data signal and couplesit via the delay circuit 236 to the latch circuit 238. The memorycircuit has an output terminal 246 that couples the data to both theinput terminals of the drivers 242 and 244. The latch circuit 238 has asecond output terminal 248 that couples the data to the test circuit240. During a normal mode of operation, the latch circuit 238 couplesthe data to the input of the driver 242 and to the input of the driver244 via switch 250 which during normal operation is closed. Both thetransistors 252 and 254 of the driver 244 are open such that the datafrom the memory circuit is coupled via the switch 250 directly to thedriver 244. As shown, during normal operation the driver 242 has an evennumber here two of inverters while the driver 244 has an odd number hereone of inverters. Thus, during normal operation, the signals DT and DCmust be complementary to one another.

The data input buffer 68 also contains a write global bus driver 600 forplacing write data onto the read global data bus 96 via bus line 97 asshown in FIG. 2. The data input buffer thus acts as a write pass throughcircuit for passing write data directly to the output buffer 98 duringeach write cycle.

As shown in FIG. 6A, data is input at terminal 27 through a standard TTLinput buffer of a type well known in the prior art. The output of TTLbuffer 27 is data complement. The data in the form of data complement isthen transferred to node 596 and, passing through inverter 598, isplaced on node 604. The data is input to NAND gate 606 andsimultaneously is presented to transfer gate 608. When the clock kSIDEgoes low, transfer gate 608 is enabled to provide the data on node 604to the input 610 of a master latch having inverter 612 and 614. Theoutput terminal 616 of the master latch is presented to the transfergate 618. A transfer gate 620 provides a feedback path from the outputof inverter 614 to the input of inverter 612. The output of the transfergate 618 is connected to the input 622 of a slave latch having inverter624, inverter 626 and transfer gate 628. The output 246 of the slavelatch is provided to the first output driver 242 and also to the secondoutput driver 244. On node 630, data complement from the first driver242 is input to NAND gate 632 and NOR gate 634. The output of NAND gate632 drives a first transistor 636 and the output of NOR gate 634 drivesa second transistor 638. These two transistors 636 and 638 comprise awrite global bus driver 600 which places the write data onto the globaldata bus 96 via write data transfer bus 97 during each write cycle.

In one embodiment, lines 29 and 31 are not connected so that delaystages 33 and 35 are used. Alternatively, lines 29 and 31 are connectedand delay stages 33 and 35 are bypassed.

In one embodiment, only data compliment is provided on the global databus 96 via terminal 34. In an alternative embodiment, both true data andcomplement data are provided, with terminals 30 and 32 connected to theGDB 96 via tristate drivers coupled to bus line 97.

The write bar signal WRTB on line 602 is input to NOR gate 640 whoseoutput is connected as the other input to NAND gate 632. WRTB bar line602 is directly connected as the other input to NOR gate 634.

The operation of FIG. 6A during a write cycle will now be described.During a write cycle, data is provided to input terminal 27 at aselected rate. During a write, signal SPOR is low and signal write barWRTB on line 602 is also low the entire time during a write cycle. Bothinputs to NOR gate 640 are low so the output of NOR gate 640 is high andone input to NAND gate 632 is high. Also, with WRTB in low, one input toNOR gate 634 is low.

When data is presented at input terminal 27, after a brief delay, thecomplement data signal is present at node 596 and data is at node 604.If lines 31 and 29 are coupled, the complement data signal will morequickly appear at node 596. When clock kSIDE goes low, the data istransferred from node 604 to node 610 and data subsequently is providedat node 616. kSIDE stays low a sufficient time to permit the data tostabilize at node 616, which usually happens very quickly. This is thefirst phase of the clock kSIDE. At the second phase of the cycle of theclock kSIDE, the clock transitions to high which disables transfer gate608 and enables transfer gates 618 and 620. At the same time, kSIDEBgoes low to disable transfer gate 608 and enable transfer gate 618 and620. With transfer gate 620 enabled, the feedback loop from latch 614 iscompleted so that the state of node 610 is held constant. The data atnode 616 complement is transferred out to node 622 and, a very shorttime later appears at node 246 as data.

The data is then presented to node 630 which is one input to NAND gate632. The other input to NAND gate 632 is high so therefore the state ofthe data other input controls the state of the output of NAND gate 632.Namely, if the data presented is a high, the output of the NAND gate isa low, which turns on transistor 636 to drive terminal 34 high to placea high on the global data bus. Conversely, if node 630 is low, then a 0is at NAND gate 632 which places a 1 on the output to turn offtransistor 636. At the same time, the 0 is presented to NOR gate 634which, since the other input is already 0, provides a 1 at the output ofNOR gate 634 to turn on transistor 638. Turning on transistor 638 placesa 0 on the global data bus at terminal 34. Thus, the state of thecomplement data, whether high or low will be transferred from the firstdriver circuit 242 onto the global data bus during a write of the data.All data written to the global data bus is actually the data complement,so it could be viewed as the global data bus bar. As is known in theart, data or its complement may easily be obtained from the other usingan invertor.

When a write is not being performed, WRTB is taken high on line 602.With one input to NOR gate 640 high, the output is always held lowregardless of the other state of the input, so that the output to NANDgate 632 is always held high to permanently turn off transistor 634. Atthe same time, one of the inputs to NOR gate 634 is held high to ensurethat the output is always low to turn off transistor 638. The writeglobal bus driver 600 is therefore disabled. No output can be providedat terminal 34 and it presents a high impedance state to any signalwhich is attempted to be provided the input. Thus, when a write is notbeing performed, line 97 is disabled to either output data or inputdata.

The write driver bus 600 has the same structure and operationalcharacteristics as the global data bus driver 95. The transistors of thewrite global data bus driver 600 are made sufficiently large to drivethe global data bus. Typically, they are approximately half the size ofthe transistors 332 and 334 of the global bus driver 95 explained ingreater detail herein. The transistors are made sufficiently large todrive the output buffer 98 with the write data while a write is beingperformed.

In summary, when an address is presented on the address bus to writedata, data is written to that address along the path as described hereinfrom the data input buffer, the write driver through the column selectand into the memory array. Simultaneously, the data input bufferprovides the write data very quickly and efficiently onto the globaldata bus 96 through the global write driver 600 from the data inputbuffer. A very simple, yet fast and efficient circuit is provided forplacing the write data on the read global data bus 96 simultaneouslywith it being written into the memory array. The write data stored inthe memory array is then input to the first stage of the output bufferso that on the subsequent read cycle, the data that was previouslywritten to the memory array is provided as the output of the outputbuffer 98 on the first read cycle.

As explained with respect to the output buffer 98, the data is outputonto data line 27 in a pipelined fashion. That is, during a read when afirst address is provided to the memory array for reading data, the datais propagated out of the array to a first pipeline stage in outputbuffer 98. The most recently written data was previously stored in thefirst pipeline state of output buffer 98 and thus, during the first readcycle the most recently written is transferred to the second pipelinestage of output buffer 98 and provided as the output on line 99. On thesubsequent read cycle, the data stored at the address presented at thefirst read cycle is moved to the next pipeline stage of the outputbuffer 98 and provided as the output and the data stored at the addresspresented at the second read cycle is placed in the first pipeline stageof the output buffer 98. During the third read cycle, the data stored atthe address presented at the second read cycle is output and so on, thegeneral concept of a pipeline output buffer being well known in the art.

FIG. 6B is a schematic diagram of one embodiment of the write driver 78of FIG. 2. The write driver 78 has an input stage 256 that has inputterminals coupled to the input terminals 40, 42, 48 and 45 as shown. Theinput stage 256 has two output terminals 258 and 260, respectively. Theinput stage is also coupled to a power supply VCC via a first powersupply terminal and ground via a second power supply terminal. An outputstage 262 has a pair of input terminals 264 and 266 that arerespectively coupled to the output terminals 258 and 260 of the inputstage 256. The output stage 262 has write power and write-complementpower terminals coupled to terminals 46 and 44 respectively of the writedriver 38. The output stage 262 also has output terminals coupled to theoutput terminals 74 and 76 of the circuit 38. A test mode stage 268 hasan input terminal coupled to the input terminal 50 and has a pair ofoutput terminals that are respectively coupled to the enable terminalsof the output stage 262, which in this embodiment are the inputterminals 264 and 266.

During a write cycle, the signal BSBT is inactive high and the signalBLKW is active low. Thus, both the input and output stages 256 and 262are enabled such that the output circuit 262 drives the lines 74 and 76with the proper signals to write the proper data value to the selectedmemory cell. For example, if the signal DC is a logic low and the signalDT is a logic high to indicate a data value of logic 1, then the outputcircuit 262 generates the signal WBC a logic low and for the signal WBTa logic high. During a read cycle, the signal BLKW is inactive high,which causes the output stage 262 to drive both signals WBC and WBT to alogic high level by coupling the terminals 52 and 54 to the WDPC andWDPT lines respectively, regardless of the state of the signals DC andDT.

Although a specific schematic diagram is shown for the write driver 75,the driver 75 may include other specific circuit arrangements to performthe same function, that is, to decouple the WDPC and WDPT lines from therespective WBC and WBT lines 74 and 76, respectively, when the writedriver circuit 75 is associated with an unselected memory block 54during a single cell test mode.

FIG. 6C is a schematic diagram of one embodiment of the column selectcircuit 78 of FIG. 2. One set of circuitry is shown for a single pair ofBLT 82 and BLC lines 80, it being understood that similar circuitry isincluded for the remaining Y-1 pairs of BLT and BLC lines. As shown, thecircuit 78 includes a pair of active load transistors 268 and 270 that,when activated, couple the associated BLT and BLC lines 82 and 80 to thepower-supply voltage VCC via a power-supply terminal 272. An equilibratetransistor 274 is coupled between the BLT and BLC lines 82 and 80. Apair of read pathgates, here transistors 138 and 134 are coupled betweenthe BLT and BLC lines 82 and 80 and the RBT and RBC lines 90 and 92,respectively. A pair of write passgates 142 and 144 couple the BLT andBLC lines 82 and 80 to the WBT and WBC lines 76 and 74 via terminal 68and 66, respectively. A column select circuit 284, which includes a NANDgate 136 and an inverter 140 receives the signal ISO via terminal 132and the column line from the column select bus that is associated withthe corresponding pair of BLT and BLC lines. For example, in one aspectof the invention, Y=15. Thus, there are 16 BLT/BLC line pairs that canbe coupled to WBT and WBC lines 76 and 74. Thus, the circuit shown inFIG. 6C is replicated 16 times, one circuit for each BLT/BLC pair. Eachof these 16 replicated circuits is coupled to a different one of the 16lines that form the column select bus. Thus, when a particular memorycell is to be either read from or written to, the circuit associatedwith the corresponding BLT/BLC line pair has its column select linecarry an active signal to couple the BLT and BLC lines to theappropriate read or write lines as is further discussed below.

The column select circuitry 78 is used for both writing data to andreading data from the memory array block 54. The circuit 74 thereforeincludes input signals WBT 76 and WBC 74 for providing data on bit linesBLT 82 and BLC 80 under control of the ISO and column select signals 130and 132. As will be appreciated, when data is being written to the arraythe data true is provided on signal line WBT 76 and the data complementis provided on signal line WBC 74. While the data is present on the databus lines, signals BLCC and EQ are held high while the signal COL 130and ISO 132 are held high. The output of NAND gate 136 is low to turn onread control transistors 138 and 134 and the output of inverter 140 ishigh to turn on write access transistors 142 and 144. Data is thuspassed from WBT 76 to BLT 82 and from WBC 74 to BLC 80 so that data ison the column lines for storing in each individual memory cell.

In operation, during a read or a write cycle, the BLL signal goes activelow to pull up the BLT and BLC lines to approximately VCC. Prior toeither a read or a write cycle, the BLEQ signal goes active low toequilibrate the associated BLT and BLC lines. The BLEQ signal then goesinactive high before the BLL signal goes active low. Also during a reador a write cycle, both the ISO and column select signals go active highto activate and thus close both the read passgates 276 and 278 and thewrite passgates 142 and 144. Such a circuit structure eliminates theneed for separate passgates for the sense amplifier 94 (FIG. 2) that iscoupled to the RBT and RBC lines 85 and 87. During a write cycle, theISO signal remains active high during the entire write cycle. The ISOsignal remains active high only for an initial period of a read cycleand then goes inactive low disabling, i.e., opening all of the passgates138, 134, 142 and 144. This decouples the sense amplifier 94 from theBLT and BLC lines after it has sensed enough of the data value stored inthe memory cell such that it can attain its steady state value morequickly because the parasitic capacitances associated with the BLT andBLC lines (and also the WBT and WBC lines 58 and 56 which are alsocoupled to the sense amp via the passgates 280 and 282) from the inputsof the sense amplifier.

As discussed above, one problem with the prior art is that a single ISOsignal and a signal column select bus were coupled to all the memoryblocks in a memory device. Thus, even in unselected memory blocks theISO and column select signals could go to active levels and thus causethe switching of the NAND gate 136, the inverter 140 and the passgates134, 138, 142 and 144. Although other circuitry disconnected the WBT andWBC lines 76 and 74, and the RBT and RBC lines 85 and 87 from theexternal circuitry so that unaddressed memory cells were not read fromor written to, the switching of the aforementioned components causedsignificant dynamic current to be drawn from the power supply.Considering the relatively large number of memory blocks present on amemory device, this dynamic current draw became a significant factor inthe power used by the memory device.

Referring to FIG. 2 in one aspect of the invention, the block read-writecontrol circuit 125 generates a separate ISO signal for each memoryblock. Thus, for all unselected memory blocks, the ISO signal remainsinactive low during both read and write cycles to prevent the columnselect circuit 78 in unselected blocks from drawing dynamic current fromthe power supply. In the embodiment, the memory device 50 of FIG. 1 thathas 32 memory blocks 14, where four memory blocks 14 are selected duringeach read or write cycle, then only four memory blocks 14 instead of 32memory blocks 14 draw dynamic current during a read or a write cycle.Thus, in such an aspect of the invention, the dynamic current drawn fromthe power supply during read and write cycles is only one-eighth thedynamic current drawn by known memory devices. Such a significantreduction in dynamic power use allows the memory device 10 to be used inlow power applications such as where the memory device 10 is powered bya battery.

Although a specific circuit is shown for the column select circuit 78 ofFIG. 2, it is understood that other similar circuitry may be used toobtain the same advantages. For example, the P-channel transistors maybe N-channel, and the N-channel transistors may be P-channel.Furthermore, a gate other than the NAND gate 136 may be used to form thecolumn select circuit 284.

FIG. 6D is a schematic diagram of the read circuit 94 and the globaldata bus driver 95 of FIG. 2.

The read circuit 94 includes a sense amplifier 290 that has a firstinput terminal 292 that is coupled to the RBT signal on the line 92 anda second input terminal 294 that is coupled to the RBC signal on line90. The sense amplifier also includes a first power terminal 296 that iscoupled to a voltage VCC and a second power terminal 298. An enablecircuit 300, which here comprises an N-channel transistor, has a controlterminal coupled to the SAEN signal via the input terminal 67 and has apair of switch terminals coupled between the power terminal 298 of thesense amplifier 290. An equilibrate circuit 302 has an equilibrateterminal 304 that is coupled to the signal SAEQ via the terminal 69. Theequilibrate circuit 302 is also coupled to voltage VCC via power-supplyterminals 306. The equilibrate circuit 302 includes a first equilibrateswitch 308 that has a control terminal coupled to the SAEQ signal and aswitchable path coupled between the RBC line 90 and the RBT line 92. Asecond equilibration switch 310 has a control terminal coupled to SAEQand a switchable path coupled between VCC and the second power terminal298 of the sense amplifier 290. A third equilibration switch 312 has acontrol terminal coupled to the SAEQ signal and a switchable pathcoupled between VCC and the RBT line 92. A fourth equilibration switch314 has a control terminal coupled to the SAEQ signal and a switchablepath coupled between VCC and the RBC line 90.

In operation, during a period of time when neither a read nor a writecycle is being performed by the memory device 50, the SAEQ signal is aactive low to cause the equilibrate circuit 302 to equilibrate the senseamplifier by coupling together the RBC line 90 and the RBT line 92 atthe input of the sense amplifier and coupling both of these lines to thepower supply voltage, here VCC. At the beginning of a read cycle, theSAEQ signal goes inactive high to disable the sense amplifier 290, andthe SAEN signal goes active high to enable the sense amplifier 290 bycoupling the power terminal 298 to ground. The sense amplifier thenamplifies the initial signals placed on the RBC line 90 and the RBT line92 to give a full signal data value on these lines. For example,depending upon the value of data stored in the memory cell that is read,one of the lines RBT and RBC will be at a logic low level, i.e., 0volts, and the other of the RBT and RBC lines will be at a logic 1,i.e., VCC volts. The amplified data value is coupled to the global databus driver 106 via the RBC and RBT lines 90 and 92.

During a write cycle, known read circuits often draw supply current fromthe power supply that provides the voltage VCC even when the senseamplifier 290 is disabled. For example, referring back to FIG. 6C,during a write cycle, the passgates 276 and 278 are closed, therebycoupling the RBC and RBT lines 90 and 92 to the BLT and BLC lines. Thus,one of the lines 90 and 92 will be coupled to a logic high level, i.e.,VCC, while the other line 90 and 92 will be coupled to ground viatransistors 142 and 144, respectively. However, because the passgates276 and 278 are P-channel, the line 90 and 92 that is coupled to groundwill actually be at one transistor threshold above ground, i.e., in oneembodiment of the invention, approximately 1.5 volts.

Referring to FIG. 6D, in known read circuits, the switch 310 has itscontrol terminal coupled to signal SAEN. With such a configuration andwith VCC coupled to one of the lines 90 and 92 and approximately 1.5volts coupled to the other of the lines 90 and 92, the sense amplifier290 may draw a supply current through the switch 310 and through the oneof the N-channel cross-couple transistors having its gate coupled to theline 90 and 92 carrying VCC. Furthermore in known read circuits, theSAEQ signal is normally active low during a write cycle to maintain thesense amplifier 290 and an equilibrated state.

Still referring to FIG. 6D, the read circuit 94 and more specificallythe sense amplifier 290, draws substantially 0 current from the powersupply during a write cycle in which the block associated with the senseamplifier is selected, thus placing VCC on one of the lines 90 and 92and a threshold voltage approximately 1.5 volts on the other of thelines 90 and 92. This advantage is accomplished by first driving theSAEQ signal in active high both during read and write cycles. In oneaspect of the invention, the SAEQ signal is only driven to an active lowlevel when the block that the read circuit 94 is associated with isunselected. Second, the switch 310 has its control terminal coupled tothe SAEQ signal instead of the SAEN signal. Thus, during a write cyclewith the SAEQ signal in active high and the SAEN in active low, thepower terminal 298 of the sense amplifier is uncovered from both VCC andground. Thus, neither of the cross-coupled N-channel transistors of thesense amplifier 290 can allow a current to flow between the power supplyvoltage VCC and the line 90 or 92 that carries the threshold voltage.Such a read circuit 88 further reduces the current drawn by the memorydevice 10 during a write cycle, and thus renders the memory device 10suitable for low-power applications.

Still referring to FIG. 6D, the global data bus-line driver 95 includesan input stage 316 that includes totem-pole coupled pair of transistor318 that have their gates coupled to the RBT line 92 via the inputterminal 93 and a second pair of totem-pole coupled transistors 320 thathave their gates coupled to the RBC line 90 via the input terminal 91.Each transistor pair 318 and 320 is coupled between ground a powerdistribution node 322. An enable circuit, which here comprises a switch324 has a control terminal coupled to the signal SAEN via the enableterminal 112, and has a switchable path coupled between the power supplyvoltage VCC and the power distribution node 322. The enable circuit mayalso include a first switch 326 having its control terminal coupled toSAEN and having a switchable path coupled between the output terminal ofthe transistor pair 318 and ground, and a second switch 328 that alsohas a control terminal coupled to SAEN and a switchable path coupledbetween the output terminal of the transistor pair 320 and ground. Anoutput stage 330 includes a pair of output driver transistors 332 and334. The output circuit 330 has a first input that is coupled to theoutput of the transistor pair 318 of the input stage 316. This inputterminal is coupled via an inverter 336 to the gate of the transistor332. The transistor 332 has a switchable path coupled betweenpower-supply voltage VCC and the GDB line 96. The output stage includesa second input terminal that is coupled to the output of the transistorpair 320. This input is coupled directly to the gate of the drivetransistor 334, which has a switchable path coupled between the GDB line36 and ground. The output stage may also include transistors 338 and 340coupled as shown, though these are optional.

The global bus driver is a tristate driver. When the sense amplifier isenabled, the global bus driver 95 drives the global data bus to placethe appropriate data value on the data bus. At the end of a read cycle,the sense amplifier is disabled and the output of the bus driver is in ahigh impedance mode as viewed from the global data bus 96. The globalbus driver is placed in a tristate mode so that it cannot drive the GDB96. The state of the GDB 96 is held at its current value by a weak latchwithin the output buffer whenever the GDB 96 is not being driven. Inoperation during a read cycle, the SAEN signal is active low, thusforcing the enable signal to supply power from the voltage VCC to thepower distribution node 322 via the transistor 324. Thus, the inputstage receives the logic levels on the RBT line 92 and the RBC line 90amplifies these values, couples these amplified values to the outputstage, which then drives the GDB line 96 with the proper data value thatwas read and amplified by the read circuit 94.

During a write cycle known driver circuits, which typically lacked anenable circuit, sometimes allowed the driver 95 to draw a supplycurrent. For example, as stated above, during a write cycle where thememory block associated with the circuit 95 is selected, one of thelines 90 and 92 will have a logic high value while the other has athreshold value in one embodiment of the invention 1.5 volts. As shown,a 1.5 volt signal applied to either the transistor pairs 318 and 320would cause the transistors of that pair to become partially active,thus drawing a current, often called a "crowbar" current from the VCCpower supply to ground. Because there are typically eight drivers 95 permemory block times 4 selected memory blocks, this causes a significantsupply current to be drawn from the power supply. Furthermore, it issometimes desirable during initial testing and burn-in of the memorydevice 10 to write to all memory locations simultaneously with a VCCvoltage of approximately 9 volts instead of the normal operating VCCvoltages of between the 3 and 5 volts. Thus, in such a burn-in mode, allof the write memory blocks are selected and with a higher voltage forVCC, the crowbar current drawn from the supply can be quite substantial.

The structure of the data bus line driver 95 substantially eliminatesthe crowbar current such that the circuit 95 draws substantially 0supply current during a write cycle when the circuit 95 is disabled. Inoperation during a write cycle, the SAEN signal is inactive high. Thisopens the switch 324 and thus uncouples both of the transistors pairs318 and 320 from the power supply. Thus, regardless of the voltage levelat the input terminals 91 and 93 of the driver 95, the transistor pairs318 and 320 are prevented from drawing supply current. Also, thetransistors 326 and 328 drive the inputs to the output stage to theappropriate levels so that both drive transistors 332 and 334 are in anonconducting state so that the driver circuit 95 drives no signal ontothe GDB line 96. Thus, the circuit 95, when disabled, drawssubstantially 0 supply current regardless of the signal that was presentat its input terminals 93 and 91.

FIG. 7 is a schematic diagram of one embodiment of the block read/writecontrol circuit 125 of FIG. 2. As stated above, in conjunction with FIG.2, in one embodiment of the invention, there is one circuit 125 for eachmemory block 14 (FIG. 1). In another embodiment shown in FIG. 8, thecircuit 24 is constructed to service two memory blocks 14. In this case,one signal for each block is generated. For example, a block write left(BLKWL) and a block write right (BLKWR) are generated, one for a firstblock, here called the left block, and the other for a second block,here called the right block. It is understood, however, that thecircuitry for both embodiments of the circuit 125 can be similar to thatshown in FIG. 7.

The circuit 125 receives a single bit test (SBT) signal, a write block(WRTB) signal, a reset block (RESETB) signal, a block select left (BSL)signal, a block select right (BSR) signal. The circuit 125 receivesother signals that are ignored here for clarity. These other signalshave little or no effect on the inventive concepts discussed herein.

As shown, the circuit 125 includes circuitry that generates an activelow BLKWL signal when the left block is selected during a write cycle.Likewise, the circuit 125 generates BLKWR when the right block isselected during a write cycle.

The circuit 125 generates BSBTL and BSBTR signals during a single bittest mode write cycle when the left and right blocks are selected,respectively.

The circuit 125 generates an inactive high logic level for the SAEQLsignal whenever the left block is selected either during a read cycle, awrite cycle or any other cycle. The circuit 125 generates an active lowlevel for the signal SAEQL under certain conditions when the left blockis unselected.

The circuit 125 generates an active low signal level for the ISOL signalwhenever the left block is unselected. The circuit 125 generates aninactive high level for the ISOL signal whenever the left block isselected during a write cycle and during an initial portion of a readcycle when the block is selected as discussed above in conjunction withFIG. 6C.

The circuit 125 generates an active high level for the SAENL signalwhenever the left block is selected and during the latter portion of theread cycle after the sense amp provider has been initially charged withthe data value as discussed above in conjunction with FIG. 6C. Thus, inone embodiment of the invention, the circuit drives the ISOL, signalactive low and the SAENL signal active high substantiallysimultaneously. Thus, the sense amplifier 290 (FIG. 6D) is not enableduntil it is initially charged with the data value from the memory celland then uncoupled from that memory cell.

The operation of the circuit 125 is described with reference to thesignals for the left block, it being understood that the signals for theright block are generated in a similar manner. Furthermore, the circuit24 can be structured with different logic gates and combinations thereofas shown and still generate the signals as described.

FIG. 8A is a detailed schematic of one embodiment of the reset controlcircuitry 84. According to the embodiment of FIG. 8A, disable logic 143has input signal lines SAENR 142 and SAENL 144 to an OR gate 146 whoseoutput is provided to a NAND gate 148. These lines 142 and 144correspond to line SAEN 67 of FIGS. 2 and 6E for a single memory block54. The signals are labeled SAENR and SAENL because they are signalsSAEN 67 from the left and right blocks 54. During a write, both signalsSAENR and SAFNL are low so that the output of NOR gate 146 is high andthe output of NAND gate 148 is high. These signals are provided topermit disabling of the reset control block under certain conditions,such as during a read, during certain test modes or at other times asdesired. In one alternative embodiment, such as that shown in FIG. 5,the disable logic need not be present.

The signal lines LC 129 and LCS 127 permit a forced control over NANDgate 150 which is part of the reset logic 128. In one embodiment, thesignal on line LCS 127 is held high at all times during deviceoperation. With LCS held high, the output of NAND gate 152 is controlledby the signal on line LC 129. The signal on line LC is connected to aclock signal which, upon going high, will force NAND gate 150 togenerate a reset signal. Under this mode of operation, the generation ofthe reset signal is controlled by a clock provided by some other circuiton or off the chip. In one embodiment, the clock is provided externally,from an external clock provided by the system or a test station. Inanother embodiment, a clock circuit on another portion of the chipprovides the clock signal for inputting to line LC 129 to generate thereset signal to terminate the write.

The generation of the reset signal causes a write to terminate andresets other circuits of the memory block to be ready for a subsequentread or write. The reset signal is input to a number of circuits on thechip, including the block read/write control and the word line and blockselect latch. When the reset signal is enabled, a write is not permittedand enabling the reset signal forces the termination of a write andprepares the circuit for a subsequent write or read, depending on thestate of other signals.

In a further alternative embodiment, the line LC 129 is connected to atest pin to force generation of the reset signal in particular testmodes. During the test mode, the signal LCS is held high so thatapplication of various test mode signals to pin LC will control thegeneration of the reset signal by NAND gate 150. When the device is notin test mode, LCS is held low at all times. Holding LCS low ensures thatthe output of NAND gate 152 is high and thus disables any signal on lineLC. With LCS held low, the output of inverter 154 is high which providesa high input to NAND gate 148 so that the state of the output of NORgate 146 can enable or disable the output of NAND gate 148 forcontrolling the generation of reset signal from NAND gate 150 from thesignals SAENR and SAENL if desired.

In a preferred embodiment, the generation of the reset signal toterminate a write during standard operation is controlled by writesimulation circuitry 156 and 158. In the preferred embodiment shown inFIG. 8A reset control circuit 84 includes two write simulation circuits,the simulation circuit 156 receiving write data from the memory arrayblock 54 on the left and memory simulation circuit 158 receiving writedata from the memory array block 54 on the right. The lines WBCL andWBTL correspond to WBC 74 and WBT 76 from the write driver 75 of theleft array block 54. Thus, the inputs are labeled WTBL and WBCL for thememory array block 54 on the left which inputs to simulation circuit 156and WBTR and WBCR for the memory array block 54 on the right whichinputs signals to simulation circuit 158. The structure and operation ofmemory simulation circuits 156 and 158 are identical and therefore onlythe structure and operation of simulation circuit 156 is described indetail; corresponding structure and operational characteristics apply tomemory simulation circuit 158.

The write data true is provided on line 160 and the write datacomplement is provided on line 162 to NAND gate 164. Simultaneously, thewrite data is provided to cross coupled transistors 164 and 168 havingtheir gates coupled to the drain of the other transistors and theirsource regions coupled together. The output node 170 is coupled to thedrain of cross coupled transistors 172 and 174. The output 180 of NANDgate 164 is connected to the gate of P-channel transistor 178 and thesource/drain node 180 of access transistor 182. The other source/drainregion of transistor 182, node 184 is coupled to the gate of transistor186 so that the state of node 184 controls whether transistor 186 is onor off. Transistor 186 has its source connected to ground throughtransistor 188. During normal operation, the gate node 192 of transistor188 is held high so that the source of transistor 186 is alwaysconnected to ground. During a test mode, a clocked control mode or aforced reset mode, LCS signal is taken high which brings node 192 low tohold transistor 188 off. This thus disables the operation of the memorysimulation circuit 156 so that the output of NAND gate 150 andgeneration of the reset signal is controlled by signal LC and cannot becontrolled by circuits 156 or 158.

The drain of transistor 186 is connected to output node 190. Node 190provides an input to NAND gate 150 for generation of the reset signal,as explained in more detail herein. The drain of transistors 194 and 196are also connected to node 190 so that memory simulation circuit 158 mayalso exert control over output node 190. The output node 180 of NANDgate 164 is also connected to transistor 198 which is coupled to VCCthrough transistor 200. Transistor 202 provides a parallel path forconnecting VCC to node 190.

The structure of memory simulation circuit 156 to generate the writetermination signal will now be described. In a preferred embodiment,write simulation 120 includes a memory cell replicate circuit. Crosscoupled transistors 172 and 174 and access transistor 182 areconstructed and connected to replicate those same transistors of amemory cell in the memory array. The transistors are preferably sizedwith the exact dimensions and positioned with the exact placement as amemory cell within the array. All the electrical connections andprocessing performed in these circuits is identical to those for thememory cells in the array. Therefore, these transistors 172, 174 and 182will exactly simulate an actual memory cell within the array on the samememory device 50. While the exact operational characteristics of amemory cell may vary from one device to another for differentmanufacturing lots, the memory cells on a single die are all constructedusing the same mask exposure and process and thus will be virtuallyidentical to each other. Similarly, transistor 182 is sized andpositioned identical to the access transistors to one bit line in thememory cell of the array. Further transistors 166 and 168 are each sizedand positioned to replicate an access transistor to the other bit lineof the memory cells in the array. Transistors 182, 166 and 168 thereforeexactly simulates the characteristics of access transistors in thememory cell array for each respective individual die.

The gate of access transistor 182 is connected directly to the samevoltage source value which drives the word lines high in the memorycells of the array. In one embodiment, this voltage level equals VCC. Inalternative embodiments, the voltage level driving the word line of thememory array may be a boosted voltage level above VCC, and for thoseinstances the gate of transistor 182 is tied to the same value as theboosted word line voltage. Preferably, the gate of transistor 182 isalways directly connected to the voltage supply line rather thandirectly connected to a word line or simulated word line. As will beappreciated, the word line is taken high to provide access to the memorycells in the individual rows. However, the word line is taken high aselected time before the data is presented to the memory cell forwriting and therefore, by the time the data to be written to the memorycell arrives, the word line is already stabilized at high. Therefore, tosimulate the time for a write to a memory cell, the gate of the accesstransistor may be considered as already on and thus the gate oftransistor 182 may be permanently tied to the appropriate high voltage.In an alternative embodiment, where simulation of word line turn oncharacteristics affecting the memory cell is desired, the gate oftransistor 182 is coupled to a word line and a word line drive circuitwhose voltage will rise and fall with the voltage of the word line inthe memory cell array itself.

Transistors 166 and 168 are also sized and designed to simulate accesstransistors within the memory array. During operation, as explainedherein, only one of the transistors, either 166 or 168, is turned on,depending on whether the data presented is low or high, respectively.The other of the transistor in the pair is held off.

Therefore, the one transistor, either 166 or 168, is acting as theaccess transistor from the bit line to the storage data node for passingdata. Turning on one of these transistors simulates taking the accesstransistor high by addressing it from the word line. In theconfiguration shown, the transistors are advantageously cross-coupled sothey may be driven by the data directly, and whichever data is high,true or complement data is acting as the word line voltage to turn theother transistor on so that low data is always being written to thestorage node 170. The cross-coupled access transistors thus provides theadvantage that only one transistor will ever be driven off at any onetime and the on transistor is being held on long before the low goingdata arrives because both WBTL 160 and WBCL 162 are held high until thedata arrives on the respective bit lines. In an alternative embodiment,an actual word line drive simulation circuit can be used for access tothis transistor as well, using the same word line voltage, whether Vccor boosted as used in this alternative embodiment for transistor 182.The memory cell replica is then constructed in such a way that the writein data is guaranteed to trigger a change in state in the memory cellreplicate. One such example of a circuit has been shown, although manyequivalent designs could be substituted therefore and still fall withinthe scope of the invention.

Within each memory block 54 there are eight pairs of signal lines WBC 74and WBT 76 and therefore, eight pairs of signal lines WBTL 160 and WBCL162. In one embodiment, if any address is written to the memory block,all eight groups of bit lines, in the memory block must be accessedbecause the memory is configured to require eight bits to be written toor read from a single memory block simultaneously and one bit line isaccessed in each group. There are 16 bit line pairs per group and thus128 bit line pairs per block. In such a design, for convenience sake,the WBTL 160 and WBCL 162 that are closest to the reset circuit 84 areused. In an alternative embodiment of the memory device 50, all writebus lines in the memory block 54 are sensed because in such analternative embodiment, any one group of the memory block could beaccessed without other groups in the memory block being written to. Amemory device 50 such as a x1, x4, x8, x16, etc., rather than thex32-part of one embodiment of the present invention could have theappropriate groupings of bit lines monitored. For example, a x1 designwould likely require sensing every bit line or the master data writelines; a x4 design would permit sensing groups of 4; a x8 design wouldpermit sensing groups of 8, etc. In this alternative embodiment, sensingevery write bus line in the entire memory block ensures that the resetcircuit 84 will receive a signal each time a write is being performed tothat memory block.

In preparation for writing data to the memory cell array 154, the datastorage node 170 of the simulated memory cell is held high and thestorage node 184 is low. Node 170 remains high and node 184 remains lowat all times during circuit operation except when a write is actuallybeing performed. Node 170 is held high because the output of NAND gate164 is low which turns on transistor 178 to provide a high logic levelto 170. This turns on transistor 174 which connects node 184 to groundto hold node 184 low. At the same time, node 180 is also held low, itbeing the output of NAND gate 164 so that no current flows throughtransistor 182 even if it is turned on. The output of NAND gate 164remains low by having both write bit true and write bit complement lines160 and 162 held high when data is not being written to the array. Thegates of transistors 166 and 168 are thus both held high. Write bit trueline 160 and write bit complement line 162 are coupled to receive theactual write data being written to the memory array itself from signallines 74 and 76 as shown in FIG. 2. When data is not being written, both160 and 162 remain high at all times.

When a write is being performed, the write driver circuit 73 receivesthe data to be written to the memory cell. At a time previous, theaddress for the individual memory cell in block 54 has been presented tothe appropriate address decode circuitry shown in FIG. 3 so that anindividual memory cell is selected. The write bit true and write bitcomplement are provided on lines 76 and 74 to the column selectcircuitry 78 for presentation to the bit lines 82 and 80 for writing tothe memory cells. Simultaneously, the data is presented to NAND gate164. When the data to be written is presented, one of the bits will golow while the other bit remains high. If the data to be stored is a low,the WBTL line 160 will go low, whereas if the data to be written is a 1,the WBCL line will go low and WBTL, line will remain high.

When either one of the write bit lines 160 or 162 goes low, this forcesthe output of NAND gate 164 high so that node 180 is pulled high. Whennode 180 is pulled high, transistor 178 is turned off to disconnect node170 from Vcc. At the same time, one of the transistors 166 or 168 isturned off because the signal connected to the gate of such a transistorgoes to ground. A low going signal on either line, 160 or 162, willcause the respective transistor. 166 or 168, to turn off while the othertransistor remains on to pass the low going voltage through to the node170. Assume, for this example, that WBTL, goes low on line 160. Thisturns transistor 168 off to disconnect the high voltage on line 162 fromline 170. However, transistor 166 was previously on and remains in theon state. Therefore, node 170 is connected to the low going voltage andbegins to approach the low voltage level 160 very quickly because it isa direct pass through connection through transistor 166. This causestransistor 174 to turn off. As transistor 174 is turned off, node 184 isno longer connected to ground. As mentioned previously, node 180 isbeing driven high by NAND gate 164. With node 180 being driven high, thehigh voltage is passed through transistor 182 to node 184. Node 184begins to go high as transistor 174 begins to turn off. Taking node 184high causes transistor 172 to turn on which will connect node 170 toground through transistor 172 and hold node 170 low and will act to pullit towards ground if it has not already reached ground. Transistor 166remains on so that the low going signal on line 160 is connected throughto line 170 so that a low logic level is maintained on all connectednodes in the memory simulation circuit.

After a certain amount of time, depending upon the switching speed ofthe memory cell replicate which simulates a memory cell at the array,the state of the data in the memory cell switches from having a highvalue stored at node 170 and a low value at node 184 to having a lowvalue stored at node 170 and a high value stored at 184. As soon as thestate of the memory cell flips, to move node 184 high, transistor 186 isturned on by node 184 going high. Turning on transistor 186 causes node190 to discharge to ground through transistor 188 so that the input toNAND gate 150 goes low.

Upon the input to NAND gate 150 going low, the output goes high togenerate the reset signal. A high going reset signal is input to theread/write control logic block to terminate writing to the memory array54 and reset other circuits for writing or reading subsequently. Thewriting to the memory array is thus forced to terminate and theswitching of other transistors takes place so that the circuits mayreset to perform another write or read as quickly as possible. Thecircuit thus provides an internal, self-timed termination of a write toa memory cell. The timing of the writing to the memory cell replicate isselected to exactly mimic the actual time needed to write data to thememory cell. A similar switching of the data stored in the memory celloccurs if the signal on line 160 is high and the write bit complementsignal 162 is the data signal which goes low so that either signal goinglow will cause the state of the data stored in the simulated memory cell156 to switch and cause node 184 to go high to generate a low goingsignal at the input of NAND gate 150 to generate the reset signal.

Each memory device 50 thus has a self-timed write interval for eachparticular memory block. The self-timing feature provides a very fastwrite, much faster than would be possible by setting some generic writetime for all memory devices 50. The termination of the write is assuredto occur as quickly as possible after a successful write and thus thedevice is very fast as compared to prior art devices and yet is assuredof always performing, a valid write and thus is very reliable.

The presentation of the write data itself simultaneously to the resetcontrol circuit 84 and to the array for writing the data provide anexact and accurate time for the start of the write to the memory cell.When the data is presented to the column select circuitry 78 it passesthrough transistors 142 and 144 for presentation to the bit lines 80 and82 within the memory array block 54. At the same time, the data ispresented to NAND gate 164 and cross coupled transistors 166 and 168which are designed to simulate the timing required for the write data tobe presented to the memory cell in the memory array itself so that node184 of the memory cell replicate and node 220 of the actual memory cellare presented with the new data at approximately the same time. Sincethe structural and operational characteristics of the access gate andthe cross coupled transistors of the memory cell and memory cellreplicate are identical to each other, the time required for change inthe state of the transistor in the actual memory cell 200 will besubstantially identical to the time required to change the state of thedata in the memory cell replicate. Therefore, the generation of thewrite termination signal on node 190 following the change of state ofdata on node 184 will be correctly timed to cause termination of thewrite after sufficient time to ensure that the write is completed to theactual memory cell but very quickly thereafter so that the circuit isimmediately reset for the subsequent cycle, whether write or read, justas quickly as possible.

FIG. 13A illustrates the timing of various signals within the simulatedmemory cell as shown in FIG. 8A. In the example shown, WBT and WBC,signal lines 160 and 162, respectively, are held at approximately 3volts at the start of a write. In the particular example shown, the databeing written to the memory cell in the array is a 0. Therefore, signalWBT falls from the high 3 volts to 0 volts as the data is presented tothe column select circuit 78. At the same time, the data is presented onsignal line 160, shown as WBT in FIG. 13 and WBTL, in FIG. 8A.Approximately 0.25-.5 nanoseconds after WBT begins to fall, node 180goes high as the output of NAND gate 164 switches from low to high.Shortly thereafter, node 170 goes from 3 volts to 0 volts as transistors168 and 178 are turned off, taking the gate of transistor 174 low,turning it off. The high voltage on node 180 begins to turn ontransistor 172 while leaving transistor 166 off to hold node 170 at alow voltage. Transistor 174 is now off and node 184 rises from 0 voltstowards a higher voltage. As node 184 reaches the turn-on voltage oftransistor 186, node 190 falls from 3 volts towards ground, as shown inFIG. 13A. As node 190 approaches ground, the output of NAND gate 150 isswitched from low to high and a reset signal is output by inverter 153.The exact slope, and timing for which the reset signal is generatedwill, of course depend upon the switching characteristics and speed ofNAND gate 150 and inverters 151 and 153, and the slope and positionshown is an example of that which could be expected from certainconfigurations.

The reset and reset bar signal are input to the block read/write controlcircuit 125 as shown in FIG. 7. In addition, the reset signal isreceived by the word line and block select latch 104. Upon receiving thereset and reset bar signals, the block read/write control circuit asshown in FIG. 7 permits the enabling of the sense amplifier anddeselects the blocks of memory for reading or writing. The reset signalis thus effective to terminate a write and prepare the circuit for asubsequent read or write under the direction of the next set of signalsto be received. After the reset signal goes high, both write lines WBT76 and WBC 74 are both brought high. Regardless of whether the data bitwritten was a logic high or a logic low, both lines are brought to highwhen the reset signal goes high. Having both WBTL 160 and WBCL 162 highcauses the output of NAND gate 164 to be driven low so that node 180falls low as shown in FIG. 13A. Shortly thereafter, node 170 is drivenhigh by the cross coupled transistors 166 and 168 both being turned onand also as a result of transistor 178 being turned on to pull node 170toward VCC through transistor 178. This causes node 184 to go low astransistor 174 is turned on so that transistor 186 is turned off by node184 going low. This causes node 190 to rise to disable the reset signaloutput by NAND gate 150. The write sense circuit 118 and writesimulation circuit 120 have now been completely reset to the originalstate. The memory cell replicate is now prepared for a subsequent writeand will switch the node 170 from high to low regardless of whether thenext bit to be written is high or low, as has been explained. Thecircuit of FIG. 8A is thus caused to be reset immediately upon thegeneration of the reset signal and the final step in the reset procedureis the disabling of the reset signal so that the circuit is againconfigured to repeat the operation.

FIG. 13A is a simulation of the operation of the reset control circuitand a memory cell in the array. Also shown in FIG. 13A is the state ofdata in an actual memory cell 200 within the array itself. The state ofthe memory cell having the true data as shown as cell true in FIG. 13and the complement is shown as cell comp. As can be seen, cell truefollows closely behind WBT and node 170 almost exactly tracks cell true.The scale divisions of FIG. 13A being shown in 0.2 nanosecond divisions,it can be seen that the simulated timing of cell true and node 170 iswithin 0.2 nanoseconds of each other and thus is a fairly accuratesimulation. Similarly, node 184 is an accurate and somewhat delayedsimulation of cell comp. As can be appreciated, it is desired to havenode 184 at least a brief period of time delayed behind cell comp toensure that the writing to the memory cell is fully completed beforenode 184 switches to the new state to cause generation of the resetsignal. Thus, having the memory replicate cell change state atapproximately the same time, slightly delayed as the actual memory cellof the array is advantageous for generating the reset signal at almostexactly the same timing that the write is fully completed of that samedata to the memory cell itself. Advantageously, this provides for a veryfast write to the memory cell because the reset signal is generated asquickly as it could possibly occur after the write is confirmed ashaving been completed. Using the data itself that is written to thememory cell as the start signal for a write thus provides significantadvantages not possible in the prior art.

FIG. 8B is an alternative embodiment of the reset control circuitry 84.The alternative embodiment includes a time control circuit 126 which inthis embodiment has two parts, a slow time control circuit 126a and afast time control circuit 126b. The time control circuit 126 permits thetime at which the reset signal is generated to be advanced or delayed asselected by the manufacturer or the user. The fast time control circuit126b contains additional transistors which advance the timing of thereset signal so that the reset signal is generated earlier in the writefor a faster termination of the write. The slow time control circuit126a contains delay gates to delay the generation of the reset signal sothat it occurs later in the write.

FIG. 9 shows an example of a fast time control circuit 126b. This fasttime control circuit 126b includes transistors 501 and 502. The drain oftransistor 501 is connected to node 190 and its gate is connected tonode 184. Transistor 502 has its source connected to node 185 and itsdrain connected to the source of transistor 501. The gate of transistor502 is connected to a fast time mode signal FTM. When the fast time modesignal is enabled, transistor 502 is turned on and effectively placestransistor 501 as a parallel current mode to transistor 186. The sizingof transistors 501 and 502 is selected to provide the desired increasedcurrent flow from node 190 to node 185 for a faster discharge of nod190. The increase in speed of the discharge rate can be selected bydetermining the sizing of the transistors 501 and 502, the largertranistor providing a larger pass to ground for the current from node190 and slightly smaller transistors providing a smaller additionalcurrent path to ground. With either design, a parallel, additionalcurrent path is provided for grounding node 190 so that node 190 reaches0 much faster than would otherwise occur without the additionaltransistors present.

The change in operational speed can be seen by viewing FIG. 13B, asimulation of respective nodes in the alternative embodiment of FIG. 8B.

The same nodes have the same numbers in FIGS. 8A and 8B and FIGS. 13Aand 13B. As can be seen, the timing for WTB and nodes 180 and 170 is thesame as in FIG. 13A. However, as node 180 rises, the additionaltransistors in fast time control circuit 126 turn on and cause node 190to begin to fall more quickly, at a much steeper slope, shown as node190F in FIG. 13B. Node 190F therefore reaches ground much more quicklyand the reset signal is subsequently generated at a more advanced stagein the timing. The reset signal causes other circuits in the memoryblock to reset as has previously been described. With the fast timercontrol circuit 126b enabled, the reset control signal is generatedearlier in the write so that an even faster write can be completed.

In one embodiment, the number of transistor pairs in parallel betweennode 190 and 185 is selected to provide the desired increased speed of awrite cycle. In a preferred embodiment, only one transistor pair 501 and502 are provided. Alternatively, for example, if two or even four pairsof transistors 501 and 502 provided in parallel with each other betweennode 190 and 185, and the large current path to ground provided by fourindividual paths all being on will significantly advance the time atwhich the write termination signal is generated. In one embodiment, eachof the transistors 501 are made slightly different in size from theother, going from a very small transistor 501 to slightly largertransistors for each of the subsequent transistors in the set. As willbe appreciated, the smaller transistors conduct just slightly morecurrent than transistor 186 alone whereas the larger transistors conducta greater amount of current much more quickly. Alternatively, thethreshold voltages or other changes can be made to the transistors sothat their turn on speed and current carrying capability is different.

Further, in this alternative embodiment, each of the transistors 502 iscontrolled by a different fast test mode signal. For example, FTM1 canprovide a slight increase in speed, FTM2 can provide a greater increasein speed, FTM3 can provide an even greater increase in speed and FTM4the most increase. When the memory is being tested, the various testcycles can invoke any combination of the parallel paths from a singlepath, such as FTM1 or FTM2, to all paths at the same time, for thefastest possible speed. At the conclusion of the test cycle, adetermination is made as to the fastest possible speed which will ensurean accurate write to the memory. For some devices, only a single smalltransistor will be added to the path provided by transistor 186 so thatonly a slight time advance is provided for the generation of the resetsignal, such as an advance of 0.1 nanoseconds. For extremely efficientdevices, the advance may become greater, such as 0.6 to 1 nanosecondgreater, by enabling the selective combination of parallel transistorpairs 501 and 502. With only a small amount of overhead circuitry, theoperational speeds of the memory devices can be selected and controlledover a large range. Preferably, such selecting and setting the memorycell is performed at the time of manufacture, as explained later herein.However, provision can be made to permit a user to select one or all ofthe various speed modes to custom select the speed of the device to adesired application.

FIG. 10 shows one embodiment of a slow time control circuit 126a. Theslow time control circuit 126a includes a NAND gate 504 connected tonode 190 and a NOR gate 512 also connected to node 190. The other inputto NAND gate 504 is slow time mode input signal 126a. A series of delayinverters 506, 508, 510 are positioned between the NAND gate 504 and theinput to the NOR gate 512.

When the slow time control mode is enabled, the STM signal to NAND gate504 is held high. Node 190 is also held high prior to a write so theoutput of NAND gate 504 is a 0 and the input, through three delay stagesto NOR gate 512, is a 1. With one of the inputs of NOR gate 512 held ata 1, the output is low so that the input to NAND gate 150 is a high andthe output is disabled. Upon node 190 going low, one input to the NORgate 512 immediately goes low because it is directly connected to node190. The other input is delayed by having to pass through NAND gate 504to transition the output from a low to a high and then successivelythrough the three inverter stages until the other input to the NOR gate512 eventually transitions to a 0. Upon both inputs to NOR gate 512transitioning to a 0, the output goes high so that the input to NANDgate 150 goes low to cause generation of the reset signal at a delayedtime after the write has started. The amount of time of the delay can beselected by inserting or removing pairs of inverters between NAND gate504 and NOR gate 512. The larger the delay, the more inverters areinserted and for a shorter delay, all but one inverter is removed. Theamount of delay can therefore be selectively controlled by amanufacturer or a user.

The slow time control circuit 126a is enabled by STM. If STM isdisabled, for this circuit being low, then the output of NAND gate 504is held high so that the input to NOR gate 512 is low. With signal STMdisabled, this is the standard state of the slow time control circuit126a. Upon node 190 going low, the signal is immediately received at theother input of NOR gate 512 so that both inputs are now low and theoutput becomes asserted high to cause the NAND gate 150 to generate thereset signal immediately. In this circumstance, the only additionaldelay which has been introduced into the circuit is the switching of NORgate 512 and inverter 514. These can be designed as extremely fastswitching circuits so that the delay from having the slow time controlcircuit 126a is almost zero when the circuit 126a is disabled.

Examples have been provided for fast time control circuit 126b and slowtime control circuit 126a, as can be appreciated, a number of othercircuits may be used to provide the same function of advancing ordelaying the timing of the generation of the reset control signal fromNAND gate 150.

The reset signal itself is not shown in FIG. 13B to avoid confusionbecause signals 190, 190F and 190S are all shown. Of course, it will beappreciated that the reset signal is generated at the same relativetiming after node 190 goes low, as shown in FIG. 13A, so that as thetransition of node 190 is advanced or delayed, the generation of thereset signal is correspondingly advanced or delayed.

In one alternative embodiment, the memory cell replicate as shown inFIG. 8B is in fact not an exact replicate of the memory cell of thearray. In this alternative embodiment, the access transistors 166, 168and 182 are made slightly larger than the accessed transistors of thearray. The cross coupled transistors 172 and 174 can also be madeslightly larger if desired. This permits a slightly faster switchingspeed of the data in the memory cell replicate. This faster switchingspeed is desired to permit a wider range for selection of the finalwrite speed for the memory cells of the device. As can be appreciated,the additional circuitry of FIG. 10, together with the switches 186 andNAND gate 150 will cause somewhat of a delay in the propagation of thereset signal after the state of the memory cell is switched. By having aslightly faster memory cell replicate, the delay caused by the switchingcircuitry, time control circuitry and reset circuitry is compensated forso that the reset signal is generated exactly at the conclusion of awrite cycle to the array and that by the time the reset signal ispropagated to other parts of the array, the write has just beensuccessfully completed. The write time interval can easily be lengthenedor shortened, as desired, using circuit 126 to select the optimum devicewrite speed.

FIG. 11 illustrates a circuit for generating signal STM or signal FTM.In the embodiment of FIG. 8B, there are two such circuits of FIG. 11 forthe entire chip. Namely, one circuit of FIG. 11 is provided forgenerating STM and another circuit is provided for generating FTM. Thecircuits are selectively enabled for the generation of either STM or FTMas explained hereafter.

The structure of the circuit of FIG. 11 is composed of a test pad mode516, a pair of inverters 518 and 519 providing one input to the OR gate520. The other input to the OR gate is provided by an inverter 522having its input coupled to a fuse which is coupled through VCC and afeedback to a transistor 524 which will hold the inverter 522 in itscurrent state. Another transistor connected to the input of inverter 522has its gate tied to the power on reset pulse signal as is transistor528 having its drain connected to the input of inverter 518 and itssource connected to ground. A similar holding transistor to transistor524 is tied between the output of inverter 518 and its input.

The operation of the circuit of FIG. 11 is as follows. During a testmode, test pad 516 has a positive voltage applied so that it is heldhigh. The input to OR gate 520 is thus held high. If the circuit forFIG. 11 is generating the signal STM, then the test pad directly assertssignal STM so that it is applied to the circuit of FIG. 8B to enable theslow time control circuit 126a. When a similar circuit is dedicated tothe generation of FTM, providing a high voltage at pad 516 causes ORgate 520 to output a high signal for generation of FTM. The signal FTMcan therefore be positively asserted on a control of the test pad 516.

When power is initially applied to this circuit, a short power on resetpulse, POR, is generated. However, POR is a relatively brief signal andthe fuse 532, if not blown, remains connected to Vcc to hold or quicklyreturn node 523 high, so that STM and FTM are not asserted. The outputof the circuitry is therefore controlled by test pad mode 516 or by thestate of fuse 532. If fuse 532 is present, the input to inverter 522 ishigh so that the output is low and the output of OR gate 520 remainslow. In this state, signal STM or FTM are disabled and the reset circuit84 operates in the standard mode as shown in FIG. 13A. If fuse 532 isblown, the input to inverter 522 is permitted to go to ground because itis not held up by VCC to the fuse 532. When power is initially appliedto the chip, the power on reset provides a brief pulse to ground theinput of inverter 522 through transistor 526. This pulls the output ofinverter 522 high. When the output of 522 is high, transistor 524 isturned on to act as a keeper to maintain the input of inverter 522 lowafter the POR pulse terminates. Thus, blown fuse 532 will cause theassertion of signal STM or FTM from inverter 520 to enable either thefast time control circuit 126b or the slow time control circuit 126a,depending on which, if either of the two circuits have the fuse blown.It can thus be seen that under a manufacturer's or user's control, thetermination timing for a write can be selectively set for eachindividual die at the time of wafer test and sort or some later timeafter the manufacture by selectively blowing or not blowing fuse 532.

A method of selecting the duration of performing a write to a memorycell will now be described. According to principles of the presentinvention, it has been established that the write cycle time can be thelimiting factor in overall device performance and device cycle time.This is particularly true in devices having a low Vcc, for example, at 4volts or lower. At such low voltages, write performance and write cycletime can generally be maintained at acceptable levels. However, the timeinterval required to complete a write may become so long that it is thelimiting factor in operational cycle time and thus strongly affectsdevice performance. By improving write cycle time according toprinciples of the present invention, this limiting factor is greatlyimproved, therefore significantly increasing those critical factors ofdevice performance.

During testing, data is written to the memory cells using the standardcircuit of FIG. 8B without either the FTM or STM asserted so that node190 goes low to generate the reset signal at the standard time in thecycle immediately after data is successfully written to the memory cellreplicate. After the data is written to the memory cell in the arrayusing the standard timing, the data is read from the memory cells of thearray. A standard tester, not shown because it is a standard device,senses whether the data was correctly written to the memory cell. Anindication is provided whether the proper data value was correctlywritten to the memory cell. This writing, reading and sensing of thedata may be performed multiple times if desired to confirm that the chipis fully operational at a plurality of memory cells. After one or moresequences of writing the data to the memory cell using the firststandard time interval, one of the circuits of FIG. 11, for either STMor FTM is enabled as part of the test. Assuming that all of the data hasbeen correctly written to the memory cell, the circuit for FTM would beenabled via test pad 516. With the signal FTM enabled, the generation ofthe reset signal is advanced in the write cycle so that the write cycleis terminated sometime, perhaps as much as one to two nanoseconds fasterthan at the standard termination time. Data is written to the memorycell using the write duration of the second time interval, such as thefaster termination mode. Data is then read from the memory cell whichhas been written at the second, faster time interval. The data is thensensed to determine whether it was properly written to the memory cellusing this second time interval. The testing circuit then outputs anindication as to whether the data was correctly written to the memorycells using the faster, second time interval. If, after numerous teststhe data is shown to have been correctly written to the memory cellusing the faster time interval, this is an indication that the memorycells are able to effectively be written to and store data at the fasterrate, and therefore the part can have a faster access time than wouldotherwise be possible. After it has been confirmed that the memorydevice 50 can indeed be successfully written at the faster timeinterval, the fuse 532 is blown to permanently enable FTM for all memoryblocks on the device 150. The device 150 is thereafter provided a writecycle rating of a faster time scale which may be 0.5 nanoseconds to upto one or two nanoseconds faster than the standard write cycle. The timeinterval for writing is thus a shorter time interval than the standarddesign for that particular chip, and the chip will have increased demandin the marketplace.

On the other hand, a similar writing, sensing and testing can occurusing the slow time mode asserted through a test pad 516, as will now beexplained. Assuming a new memory device 50 is being tested, data iswritten to the memory cell using the write duration of the standard timeinterval with termination by the reset signal as shown in FIG. 13A. Thedata is then read from the memory cells, and sensing is performed todetermine whether the data was correctly written to the memory cells.For some memory devices 50, there may be just sufficient minorvariations in the standard processing lot that, due to an increase incontact resistance, variations in threshold voltage, slight incursionsby the bird's beak of the oxide into the active area, or any othernumber of changes, that writing to the memory cell may not have beencompleted when the reset signal has generated. The generation of thereset signal will automatically cause termination of the write to thememory cell. If the write has not been completed, the proper data willnot be written to the memory cell, and the reading of the data willindicate that the data has not been correctly stored. The chip is nowtested using a second time interval which, in this instance, is thelonger time interval. This is done by putting the high voltage on testpad 516 for the circuit which generates the slow time mode signal toenable circuit 126a. With circuit 126a enabled, the reset signal isgenerated at a later point in time in the write so that more time isprovided for the actual writing of data to the memory cells in thearray. This reset signal can be between 0.5 up to one, two, or possiblyfour or more nanoseconds longer than the standard time for generatingthe reset signal as has been explained with respect to FIG. 10. Data isnow written to the memory cells using the second, slower time intervalin the slow time mode. The data is sensed from the memory cells todetermine whether it was properly written during this slower timeinterval. An indication is output showing whether there was success inwriting to the memory cells using the slower time mode. For many memorydevices 50, the slower time mode will permit the data to be accuratelywritten to the memory cell, even though it could not be written usingthe standard time interval. For those memory devices 50 that, aftersubsequent testing, show that they perform properly at the slower timemode, the fuse 532 is blown for the slow time mode circuitry topermanently enable the slow time mode. The speed of the device is thusrated as somewhat slower, depending on the speed of the slow time modecontrol circuit 126a. For example, it may be rated as having one 0.25,0.5 or more nanosecond longer write cycle. However, the chip still hassignificant value in the commercial marketplace and can be sold at theproper write cycle rating. Previously, such chips had to be discarded atgreat cost to the manufacturer. With the advantages of this invention, alarge number of chips which otherwise were discarded can now be shippedto customers, the only difference being that they are rated at asomewhat slower write cycle, but still having significant commercialvalue.

This invention has particular merit if a number of redundant rows andcolumns have been invoked. There are some circuit designs on the marketwhere the invoking of redundant rows and columns causes a very slightdecrease in the speed at which data can be written to or read from in agiven memory cell on the chip. Unfortunately, in the prior art, thiscould cause the speed ratings of the chips to suffer greatly, or perhapseven cause some chips to be discarded which were otherwise operationalmemory devices. According to the principles of the present invention, ifsuch memory device 50 is fully operational but is merely slightly slowerbecause of some process variation, or having redundant rows or columnsinvoked, the device will still have significant commercial value and canbe sold on the market rather than being discarded.

In an alternative embodiment of the present invention, the time modecircuitry is selected and enabled through various other techniques. Forexample, instead of blowing a fuse, a final metal level can be used todetermine which speed circuit is enabled for writing to the memory cell.In an alternative design, an external pin is provided connected to oneor both of the test pads 516. The test pad 516 can be accessed via anexternal pin after the memory device 50 has been enclosed in a package.During a test after packaging, various speeds can be asserted using one,two or additional access pins. The pins connected to the test pads 516can have other functions during normal chip operation and are used onlyin the test mode. The appropriate speed can be selected by placing ahigh voltage on the appropriate pin to permanently disable orselectively enable the particular speed mode desired. In thisalternative embodiment, the pins connected to the pads 516 can be madeavailable to the user. This permits a user to selectively test the writetiming cycle. In one alternative embodiment, the fuse 532 is coupled soit is blown by an overvoltage from an outside pin, thus permitting auser to select the write time interval. The user, of course, will needto be sophisticated enough to ensure that the write cycle is always madesufficiently long that data is stored during the write if they elect totry and involve changes in the writing timing.

A significant advantage of the present invention is the modificationswhich can be made to the memory circuit before and after a burn-incycle. The write speed of the memory can be tested before a burn-incycle. After a burn-in cycle, the write speed can be tested again todetermine whether the chip is still able to operate as fast as beforethe burn-in cycle. The stressing during the burn-in cycle may causesufficiently slight variations in the memory cell array that it will notoperate in the fast time mode which it was capable of operating inbefore the burn-in cycle. The standard time mode or slow time mode cantherefore be selected as a time interval for the write cycle after aburn-in has been completed rather than discarding the memory device 50.

A further significant advantage and use of the present invention is tofind potentially weak bits under test. Assume, for example, that allmemory cells pass the standard speed test which is the standard for thepart specification. In this circumstance, the part can be marketed andsold as a full speed access part. However, as an additional testfeature, the fast time mode is invoked so that a write is performed atthe fast speed time interval. The results of this write are sensed andcompared to determine whether all bits were successfully written. In theevent a few bits were not successfully written, these bits can then bereplaced with redundant rows and columns.

The part is then tested again at the high speed test mode to ensure thatall memory cells can be written to even at a speed beyond the maximumrated speed for the device. Preferably, such testing and replacing ofredundant rows and/or columns occurs during wafer sort. The device isthen sold as a standard speed device. Even though all memory cells inthe array operate at the standard speed device, by stressing the chip ata high speed mode, those potentially weak cells have been identified andeliminated from the array by being replaced with redundant memory cells.The memory device 50 therefore has a much higher reliability than othermemory devices on the market because all memory cells have a much highermargin for performing a proper write of the data under the time andvoltage parameters of standard operation. Again, this particular testmode and replacement assumes that all memory cells of the array meet thefastest possible specifications at which the chip will be sold. The chipis then tested at one speed faster than the fastest possible write timefor the specification sheet. Those memory cells which passed the priorspeed test but failed the fastest excess speed test are replaced to weedout potentially weak memory cells.

FIG. 12 shows an alternative embodiment for a circuit which determinethe start of the write. The write sensing circuit 118 of FIG. 5A isshown in a first embodiment of FIG. 8A to include signal from WBT 76 andWBC 74 being input to a NAND gate 164 and to cross coupled transistors166 and 168. FIG. 12 is an alternative embodiment for such a writesensing circuit 118 of FIG. 5.

According to the alternative embodiment of FIG. 12, the actual bit linesthemselves from the memory array are connected to control the gates oftransistors. In the embodiment shown, the transistors are P-channeltransistors 534-539. When the PRE signal is enabled, node 540 is pulledto ground, and then the weak latch 542 holds node 540 at ground eventhough PRE goes low. With all bit lines being held high, both the bitline and bit line bar are in the state before a write or a read, thenode 540 remains low. When a write is performed to the memory array, onebit line in the pair being written to will be driven low, and the otherbit line will be held high. As soon as one bit line in the pair goeslow, the P-channel transistor is turned on connecting node 540 toground. The direct current flow through the respective P-channeltransistor tied to VCC is more than enough to overcome the weak latch542 to drive node 540 high. Node 540 is now being driven high as adirect result of a write on the bit line in a memory cell within thearray itself. The output of node 540 then passes through an inverter 544which is driven low to drive node 170 low as shown in FIG. 8A. With node170 driven low as shown in FIG. 8A, the state of the memory cell isswitched from high to low so that node 184 is driven high to causetransistor 186 to turn on as has previously been described. The resetsignal is then generated by NAND gate 150 with the subsequent resultthat the write is terminated. The PRE signal is again asserted to bringnode 540 low to reset the memory replicate circuit to having a highstored at node 170 and having all bit lines pulled high.

In this particular alternative embodiment, an entire additional row ofP-channel transistors is required for one group of bit lines in eachblock for directly sensing the writing of data from the bit line withinthe memory array itself. This provides the advantage that the timing isdriven directly from the data on the bit line itself with a veryaccurate simulation of the time required for writing data to each memorycell.

The advantage of the embodiment of the circuit of FIG. 12 has thepotential for an increased accuracy with respect to writing to thememory cell itself. However, it has the distinct disadvantage that alarger number of transistors is required together with the additionalconductors for node 540 and the additional conductive line to node 170.Thus, while the circuit of FIG. 12 is one preferred embodiment, thecircuit of FIG. 8A in which the write sensing circuit is comprised of aNAND gate 164 and cross coupled transistors 166 and 168 designed tosimulate the access time to a memory cell is also a preferredembodiment, and in many circuits will be preferred due to its simplicityand accurate simulation of writing data to the memory cell. As will beappreciated by those of skill in the art, many equivalent write sensingcircuits can be designed which sense when data is presented for writingto the memory cell. One of the significant advantages of the inventionis that it is the writing of data to the memory cell which starts thetiming of the memory replicate circuit for generation of the resetsignal. Any equivalent circuits which use the data itself to start thetiming of a write may be considered to fall within the scope of theinvention.

The write simulation circuit 120 has been shown in the embodiment ofFIG. 8A as being the exact same memory cell as used in the array. In theembodiment shown, the memory cell is an SRAM memory cell having crosscoupled storage transistors whose storage node is maintained by apolyresistor. As will be appreciated, if the memory cell is a full CMOS,6-Transistor memory cell within the array, the write simulation circuit120 would include a memory cell replicate which is also a full CMOS,6-Transistor memory cell. Similarly, if the memory cell in the array isan EEPROM memory cell or a flash EEPROM memory cell, the writesimulation circuit would include a circuit that simulates the timerequired to write to such a memory cell. Preferably, it would include amemory cell replicate of the very type in the memory cell array, such anEEPROM memory cell, to exactly simulate the write time for a memory cellin the array.

FIGS. 14 and 15 illustrate systems which include a memory device 50 thatincorporates the invention. FIG. 14 shows a memory 50 within a computersystem 560. The computer system includes a microprocessor and otherinput/output devices. In a preferred embodiment, the computer system 560includes a full 32-bit high speed microprocessor, such as those beingsold by Intel, Motorola, IBM and other companies. The computer systemmay have connected thereto various input devices 562 and output devices564. Additional data storage devices 566 which may include hard drives,CD ROMs or other off-site storage are also connected to the computersystem 560. Using the high speed write memory device 50 of the presentinvention, the computer system 560 is thus able to write to andtherefore perform all other operations much faster than was previouslypossible with other standard memory devices.

FIGS. 15A and 15B show additional specific uses for one embodiment ofthe memory device 50 according to the present invention. In oneembodiment, the memory device 50 is a 32K×32-bit synchronous pipelinedBurst SRAM, called a BRAM. It has pipelined output and can operate as aCMOS cache Burst SRAM.

Burst operations can be initiated with either ADSP (processor addressstatus) or ADSC (controller address status). The burst advance inputADV, allows the next burst address to be generated internal to the BRAM.

Cache burst read cycles are initiated with ADSP, without regard to ADSCor BWE, using the external address clocked into the on-chip addressregisters when ADSP is sampled low. All Chip Selects must be assertedfor ADSP to begin the burst cycle. The output buffers will be enabled byOE when the BRAM is selected. If the device is going from a deselect toa select mode, the device will be selected and the outputs enabled onthe following clock cycle. In a read operation, data accessed by thecurrent registered address will be available tKQ from the next risingclock edge in a pipelined fashion.

The ADV input is ignored on the clock edge that samples ADSx asserted,but is sampled on all subsequent clock edges. The address is incrementedinternally to the BRAM for each read burst access where BWE and GW aresampled high, ADV is asserted low, and both address strobes are high.Data is always valid at tKQ for all Outputs (DQ0-31) from the rising ofclock (K).

The ADV input (burst address advance) provides control of the burstcounter. The ADV input controls subsequent burst data accesses after thefirst data of the burst cycle is processed. Each time ADV is active lowfor subsequent bursts at the rising edge of the clock input, the burstcounter is advanced to the next burst address. The address is advancedbefore the operation. The BRAM will suspend the address burst sequencewhen the ADV pin is high during positive clock transitions. Uponcompletion of the full internal burst count, the address willwrap-around to its initial base address. The logic state of the LBOinput determines the burst sequence as interleave (i486™ or Pentium™ forIntel bursts) or linear for other processors (RISC, PowerPC, Cyrix M1).

Write cycles are performed by disabling the outputs with OE prior toasserting BWE. A global write enable (GW=low) writes all 32 bitsregardless of the state of BWF or individual byte write select inputs.When GW is high, one or more bytes can be written by asserting BWE andindividual byte write selects (BW1-4). A byte write table (not shown)lists which byte write selects controls DQ0-31. BWE is ignored on risingclock edges that sample ADSP low, but is sampled on all subsequentrising clock edges. Output buffers are disabled tKQHZ after K when BWEor GW is sampled low (independent of OE). Data is clocked into the datainput register when a proper write operation is implemented. The writecycles are internally self-timed, and are initiated by the rising edgeof the clock input. A write burst cycle continues with the addressincremented internal to the BRAM when BWE and ADV are sampled low at thenext rising clock edge.

For one embodiment of the memory device 50, connected as shown in FIGS.15A and 15B, Read or Write operations can be initiated with ADSC insteadof ADSP. The differences of these inputs are noted as:

1. ADSP must be high when ADSC is asserted low to initiate a cycle withADSC.

2. All Write Enable signals are sampled on the positive going clock edgethat samples ADSC low (with ADSP high).

3. ADSP is blocked when CE1 is high. The memory device 50 can beselected with either ADSP or ADSC, but can only be deselected with ADSCwhen CE1 is high.

The device of FIG. 15A includes two memory devices 50 connected toPentium processor to provide 256KB cache SRAM. The Pentium chip 570,working with cache controller 572, is able to have full 32-bitread/write access from each of the memory devices 50. The memory devices50 include the self timed high speed write internally and thereforepermit faster writing to the respective memory devices under the controlof the Pentium 570 and cache controller 572.

FIG. 15B illustrates the computer system having a 512KB cache comprisingfour memory devices 50 connected to the Pentium 570 and cache controller572 to provide additional data storage and burst operational capability.The memory device 50 thus provides an architecture for building a32K×64-bit burstable L2 data cache SRAM array (256K bytes) by using onlytwo (2) devices, as shown in FIG. 15A. Four (4) devices are used toprovide a 512K byte cache, see FIG. 15B. The memory device 50 has threechip enables for easy depth expansion. The chip enables are registeredto allow contention free operation when implementing a 512K byte,dual-bank cache configuration.

The invention has been described, including a large number ofembodiments and alternative embodiments which may be used to implementthe invention. As will be recognized, one embodiment of the inventionincludes an SRAM memory device. Any equivalent circuits which perform asimilar function may be used and substituted for those described hereinand thus fall within the claims of the present invention. The claims aretherefore not limited by the description provided herein but are broadenough to cover and include alternative embodiments in equivalentcircuitry for carrying out the claim features.

I claim:
 1. A circuit for generating a write termination signal,comprising:a start write sensing circuit for sensing the start of awrite; a write termination signal generation circuit for generating asignal to terminate the write; and a timer circuit coupled to the startwrite sensing circuit and coupled to the write termination signalgeneration circuit, the timer circuit including a memory cell replicatehaving a pair of transistors of the same sizing and in the same circuitconfiguration as a memory cell in an array of memory cells; and aswitching circuit coupled to the timer circuit.
 2. The circuit accordingto claim 1, further including:a state change generation circuit coupledto the start write sensing circuit for forcing a change in the state ofthe memory cell replicate upon the write to the memory cells in thearray being sensed.
 3. The circuit according to claim 2 wherein theswitching circuit is coupled to the memory cell replicate to generate aswitching circuit upon the state of the memory cell being changed. 4.The circuit according to claim 1, further including:a variable timecontrol circuit coupled to the switching circuit.
 5. The circuitaccording to claim 4 wherein the variable time control circuit includesa time advance circuit to advance the time for causing the writetermination circuit to generate the write termination signal from afirst selected time to a second selected time.
 6. The circuit accordingto claim 5 wherein the time advance circuit includes a plurality oftransistors having their gates coupled to a common switching node andtheir sources coupled to a first node and their drains coupled to asecond node to cause the first and second nodes to be coupled togetherwhen the common switching node is enabled.
 7. The circuit according toclaim 6 wherein the number of transistors in the plurality is selectedto increase the speed at which current flows from the first node to thesecond node to advance the second selected time a predetermined amount.8. The circuit according to claim 7 wherein said first node is groundand the second node is a voltage value above ground before the commonswitching mode is enabled to connected the second node to ground whenthe common switching mode is enabled.
 9. The circuit according to claim4 wherein the variable time control circuit includes:a time delaycircuit to delay the time for causing the write termination circuit or asecond selected time or generate the write termination signal at a firstselected time to a third selected time.
 10. The circuit according toclaim 9 wherein the time delay circuit includes:a plurality of delaygates to delay the output of the signal.
 11. The circuit according toclaim 10 wherein the number of delay gates in the plurality is selectedto delay the output a selected time value.
 12. A method for generating awrite termination signal, comprising:sensing the presentation of datafor writing to a memory cell; simulating the time period required forstoring the data in the memory cell; generating a write completionsignal after the simulated time period has passed; and generating awrite termination and reset signal a selected time period after thegeneration of the write completion signal.
 13. A method for generating awrite termination signal, comprising:sensing the presentation of datafor writing to a memory cell; storing a change of data state in a memorycell replicate to simulate the time required for writing data to amemory cell; generating a write completion signal after the data haschanged state in the memory cell replicate; and generating a writetermination and reset signal a selected time period after the generationof the write completion signal.
 14. The method according to claim 12,further including:simultaneously sensing the state of data on a writebus true and on a write bus complement; and generating the write startsignal when the state of the data changes on either a write bus true orthe write bus complement.
 15. The method according to claim 12 whereinsaid step of simulating the time required for storing data in the memorycell comprises:writing new data to a memory cell replicate having thesame electrical connection, device properties, and device sizing as thememory cell in the array.
 16. The method according to claim 12, furtherincluding disabling a word line selection circuit to a memory arraycomprised of memory cells and resetting a read/write control circuitunder the control of the reset signal.
 17. A computer system fordetermining the length of a write cycle comprising:a microprocessorsystem; a memory device electrically connected to the microprocessorsystem and performing a write operation under instructions from themicroprocessor; a start write sensing circuit within the memory devicefor sensing when data is provided from the microprocessor to the memorydevice for writing to the memory device; a write time duration circuitcoupled to the start write sensing circuit and generating a writecompleted signal, a selected time after a start write signal isgenerated by the start write sensing circuit; and a write terminationand reset circuit connected to the write simulation circuit forgenerating a signal to disable access of write data to the memory array.